The present invention relates to a method for the preparation of a substrate plate of a ferroelectric material or, more particularly, to a method for the preparation of a high-quality substrate plate of a ferroelectric material in a high yield from a wafer of the ferroelectric material by removing the strain produced therein by mechanical working.
As is known, several ferroelectric single crystals such as lithium tantalate LiTaO.sub.3, lithium niobate LiNbO.sub.3 and the like are prepared by the so-called Czochralski method and wafers obtained by slicing these single crystal boules are useful as a material as a crystal for laser beam modulation or as a material used in various kinds of electronic instruments including sensor elements.
When a wafer of such a ferroelectric single crystal is used as a substrate for an electronic element such as a substrate for SAW (surface acoustic wave) filter elements and the like, the wafer as sliced from a single crystal boule by use of a diamond blade must be finished on both surfaces to have adequate surface roughness by mechanical means. That is, one of the wafer surfaces is finished by mirror-polishing while the other surface should be imparted with an adequate roughness by lapping with an object to remove bulk waves. In compliance with the recent trend that the frequency range in which SAW filter elements are used is expanding toward high frequency region and smaller and smaller elements are used in electronic circuits, the requirement for the mechanical finishing of such a substrate plate has become extremely severe.
The conventional methods for finishing the surface conditions, i.e. mirror-polishing on one surface and lapping to impart adequate roughness to the other surface, are as follows. That is, a wafer of lithium tantalate or other ferroelectric material prepared by slicing a single crystal boule thereof is first mirror-polished on one surface and then lapping is undertaken on the other surface to impart an adequate roughness. This procedure of surface finishing is defective in several respects. For example, (1) the warping of such a finished wafer is as large as 80 to 120 .mu.m (see Japanese Patent Kokai 53-288), (2) the procedure of working is necessarily complicated because lapping or polishing on one surface is usually carried out by holding the wafer as adhesively bonded to a holder plate so that the procedure involves the steps of adhesive bonding and peeling after completion of finishing of the surface, (3) a wafer as sliced from a single crystal boule has a large surface roughness as well as poor parallelism between surfaces, i.e. large difference in thickness from portion to portion, so that a long working time is required to finish the surface into a mirror-polished condition by removing the strain in the surface layer produced by the mechanical working and (4) the wafer is sometimes cracked during polishing owing to the warping or strain of the wafer (see Japanese Patent Kokai 54-894). These disadvantages result in decreased yield of satisfactorily finished wafers and low productivity of the working with consequent decrease in the yield of the devices made of the ferroelectric material.
Alternatively, an improved procedure has been proposed for the surface finishing of a wafer of this kind in which a wafer obtained by silicing of a single crystal boule is subjected to lapping on one surface only followed by chemical etching so as to remove the strain produced by mechanical working with the ruggedness of the surface retained as such (see Japanese Patent Kokai 54-11693). This method is indeed effective in reducing the warping of the wafer, i.e. the first of the above mentioned four disadvantages, to about 10 to 15 .mu.m but quite ineffective in respect to the disadvantages (2) to (4) above.
Further alternatively, a method is proposed in Japanese Patent Kokai 54-894 according to which a wafer obtained by slicing a single crystal boule is, without subjecting to lapping, coated and masked with a protective film and then subjected to etching. This method is effective to given improvements in (1) and (4) of the above mentioned disadvantages (1) to (4) although this method involves another problem of the very troublesome step of chemical etching.
Moreover, the mirror-polished surface and the other surface imparted with ruggedness by lapping are finished one after the other so that improvements in the parallelism between the surfaces of the wafer and the flatness of the mirror-polished surface are limited with unsatisfactory accuracy of surface conditions notwithstanding the very severe requirements for the manufacture of SAW filter elements in high-frequency use suitable for modern communication instruments.